Press insert



March 3, 1970 Pi of D a.

J. K. BARRY mass-1:15am

Filed June 5. 1968 lill Fig. 3

INVENTOR. John K. Barry BY ATTORNEYS.

United States Patent 3,498,353 PRESS INSERT John K. Barry, Springfield, Pa., assignor to Southco, Inc., Lested, Pa., a corporation of Delaware Filed June 3, 1968, Ser. No. 734,087 Int. Cl. F16b 39/00 US. Cl. 15141.73 2 Claims ABSTRACT OF THE DISCLOSURE FIELD OF THE INVENTION This invention relates to sheet metal fasteners and particularly to press inserts.

DESCRIPTION OF THE PRIOR ART In the prior art, most of the sheet metal fasteners of the high-strength insert type are threaded on the ouside diameter and require that the installation hole be drilled and tapped to receive the insert. An exception is the highstrength insert described and claimed in US. Patent 3,198,231, granted to Robert H. Bisbing and assigned to the assignee of the present application. The Bisbing insert utilizes oppositely directed diagonal knurls and can be pressed into an unthreaded hole. Other prior art inserts utilize knurls of various kinds, but these inserts are i of low retention strength. Still other prior art inserts are externally barbed but these inserts must be expanded to press the barbs into the wall of the hole.

SUMMARY OF THE INVENTION The present invention discloses an internally-threaded externally-barbed high-strength non-expandable hardened steel insert which need only to be pressed into a drilled or cast hole to be installed and ready for use. The new insert relies, for its high retention properties, on the fact that all sheet metal or other metal materials, into which the insert would be pressed, have some degree of resiliency, and when the force applied by the barb, during insertion of the insert, is removed, the material will rebound at least to some degree.

The insert of the present invention employs a large number of tiny barbs so that a spring-back of the deformed material of as little as .001" is all that is needed to achieve the necessary retention. I have discovered that the elfectiveness of the tiny barbs is greatly increased if the approach angle of the barb to the wall of the hole is in the range 15-25, preferably 22.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a press insert embodying the present invention;

FIG. 2 is a view illustrating the use of the insert, showing the insert pressed into a drilled or cast hole in a body of material and having a screw threaded thereinto to retain a faceplate; and

3,498,353 Patented Mar. 3, 1970 FIG. 3 is a greatly enlarged view of the barbs in the rectangle III shown in dot-and-dash lines in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawing, FIG. 1 shows an insert 10 having an internal threaded bore 15 and a large number of tiny external circumferential barbs 20. The head portion 30 is shown to be knurled with knurls, but this knurled portion is not needed, so far as the present invention is concerned. In some cases, it will be preferable to eliminate the knurled head 30 and to extend the barbs 20 for the full length of the insert 10. However, the knurled-head insert 10 illustrated in FIG. 1 has a sufficient number of barbs to provide a suflicient holding power to give the insert a retention strength greater than that of the 75,000 psi. type steel screws normally used with it, such as is illustrated in FIG. 2 by the screw 35. The knurled head 30 when used provides resistance against torque forces and in this way contributes to preventing the insert from becoming loose.

The novelty in the insert of the present invention resides in the number, the size, and the shape of the barbs 20. In brief, a large number of tiny barbs having an approach angle in the 15 to 25 range are used to provide high resistance to pull-out.

All metallic materials into which the insert of the present invention would be inserted have some resiliency, including cast iron. If a compressive force be momentarily placed against a spot on such material suflicient to deform the material, then when the force is removed, the material in the deformed spot will rebound, at least to some degree. I have discovered that a rebound as small at .001 is all that is needed for my purpose. Even cast iron provides this amount of rebound, when deformed .006" or more from its free position.

I have discovered that if the tiny rebound or springback referred to above can be caught behind a sharp strong barb, the tendency will be for the barb to penetrate the material, and if a load perpendicular to the barb penetration is applied, the barb will tend to peal the material in a shearing action. Thus, a small shear resistance to pull-out can be created. Since each barb in the insert of the present invention is tiny and only working on a penetration of .001" to .003" into the wall of the hole, the barbs can be numerous. I have discovered that an insert with a large number of very small sharp barbs closely spaced will develop a very high resistance to pullout because of the shear resistance per barb multiplied by the number of barbs.

To summarize the foregoing, the insert of the present application is novel in that it utilizes a large number of shallow closely-spaced barbs with very sharp edges to provide high resistance to pull-out. Its action depends on the inherent spring-back or rebound of all materials into which it would be inserted, to provide at least .001" of material behind each barb. Each of the barbs bites into this material and starts a shear effect when a pull-out force is applied to the insert.

I have further discovered that the insert described above is depnedent for its effectiveness also on the approach angle of the barb to the wall of the hole. I have discovered that the technique does not work satisfactorily if the approach angle of the barb exceeds 25 For example, if the approach angle be progressively increased from 25 to 45, the installation force rises rapidly as the angle increases and the barbs have greater and greater tendency to break down. Moreover, the spring-back characteristic of the hole material is greatly reduced. It appears that the steeper approach angle produces to much of an axial thrust component which causes an axial flow or parallel movement of the wall of the hole, in contrast to a radial expansion of the hole and spring-back of the material. On the other hand, if the approach angle of the barb is less than 15, the barb does not penetrate as well and the pull-out strength decreases. I have found that an approach angle range of between 15 and 25 is the most effective. An approach angle of 22 seems to be optimum because it gives the smallest barb which is practical to manufacture (.006" high and wide) with an increment which is easy to inspect, and it gives maximum or optimum barb sharpness. The back surface of the barb is approximately perpendicular to the axis of the insert.

I have found by experiment that a good compromise be a hardenable steel to provide sharp edge barbs that are strong and tough and will withstand the installation forces that tend to bend over or break off the barb edge. Hardenable material is also needed to provide strong wear-resistant internal threads.

The hole into which the insert of the present invention is to be inserted is either drilled or cast. The diameter of the hole with respect to the diameter over the barbs of the insert should be such that the passage of the barbs through the hole in the material sets up a compressive stress sufiicient to cause the material to press back upon the barbs forcefully enough to cause barb penetration into the material. A stress .of approximatley 100,000 p.s.i. accomplishes this. This stress must be varied in some materials depending on the proximity of the hole to the edge of the material, or the degree of softness and resiliency of the material, or the brittleness of the material. For example, the hole would have to be larger in cast iron than in malleable iron or steel, and would have to be smaller in zinc die castings and some aluminum castings than in malleable iron or steel.

I have found by experiment that a good compromise for most materials, and a starting point for all materials, is a hole made by that standard drill nearest in diameter to the root diameter of the barbs on a particular size of insert. This causes a .006" theoretical radial compression of the material in the wall of the hole, which in reality works out to about .004" maximum radial compression due to the tolerance on the drilled hole, slight diametral compression of the insert, and slight break-down at the barb tip. In malleable iron, this radial compression amounts to about 100,000 p.s.i., and in steel to about 120,000 p.s.i. In aluminum, it is only about 40,000 p.s.i., which is not too good, and the hole size should be decreased. In cast iron, it is about 65,000 p.s.i., which is too high for such brittle material, and the hole must be enlarged.

Tests have indicated that the insert 10 illustrated in the drawing, for screw sizes 632 through 7 -18, will develop a tensile pull-out strength of approximately 200 lb. for each lienar inch of barb present on the surface of the insert when the insert is installed properly in materials having approximately 50,000 p.s.i. ultimate tensile strength, such as die-cast aluminum alloy 360, or standard ferritic malleable iron, or 2011-T3 alloy wrought aluminum bar. To insure that the inserts 10 will be stronger in pull-out strength than a 75,000 p.s.i. steel screw 35 and to allow for varying conditiosn of installation, a minimum safety factor of 50 pounds per linear inch of brab is applied, and the inserts are designed to have suflicient barbs on the surface that the ultimate tensile strength in pounds of each size screw used would not create a load on the barbs in excess of 150 lb. per lineal inch. For practical reasons other than strength, such as desirable length of screw engagement, functional length 4 of pilot and knurl sections, etc., the actual number of barbs exceeds the theoretically necessary minimum and the load per linear inch equal to the breaking strength in tension of a 75,000 p.s.i. screw works out in practice somewhat as follows:

From the explanation given hereinabove, it will be seen that the total number of barbs present on the insert must be such that, for the screw size used therewith, the breaking strength of the screw must not exert more than lb. per linear inch of engaged barb. The word engaged is important here because it would seem that as the insert diameter is increased for a given screw size, the number of barbs could decrease until, ultimately, a large enough insert would need only one barb. In practice this is not true, and the most effective insert is the one smallest in diameter with the most barbs. There are many reasons for this, and they all bear on the fact that all barbs do not equally share the load throughout their length all the time. Because of roughly drilled holes, elliptical hole shapes, variation in resilience over the sur face of the hole, of the fact that some barbs are not as sharp as others, not truly axial alignment of the insert a as it is pressed into the hole, not truly axial tensile load on the screw, dirt in the barbs, and many other reasons, portions of some barbs do not penetrate and so do not carry their share of the load. With many barbs present, enough engagement always occurs to carry the load and the engagement is equally distributed around the circumference of the insert.

The following is a table setting forth, for different internal thread sizes (to accommodate different sizes of screw) lengths of insert, the pilot diameter, the diameter over the barbs, the number of barbs, and the diameter of the installation hole. The approach angle for each barb is 22 and the back surface is approximately perpendicular to the axis of the insert. The insert material may be 15-5 ph stainless steel case hardened by nitride process at 1050 F. to .004"-.006 case depthRC 67 hard and RC 36 core, with a Passivate finish. Alternatively, the material of the insert may be a leaded RY case or leaded C1117 cold rolled steel, case hardened to .005"-.008 depth for maximum hardness with flash cadmium plate finish to .00015" maximum thickness.

Major diameter Installation Length of Pilot over hole Inter. thread insert, No. of diameter, barbs, diameter, size inch barbs inch inch inch It will be seen from the foregoing table that for the larger sized inserts (as evidenced by the larger thread sizes) the length of the insert is increased so as to allow for a larger number of barbs. Each barb size, however, remains about the same.

In FIG. 3, barb dimensions and barb angles are indicated. These values are preferred values, but the insert of the present invention is not limited to these exact values.

The term approach angle as used in the appended claims refers to the angle between the axis of the insert and the inclined surface of the barb which is divergent outwardly from the body of the insert in a direction toward the head of the insert.

What is claimed is:

1. An internally threaded press insert of case hardened steel for installation in a hole in softer metal, said insert having:

(a) a foot end for insertion first into the hole;

(b) a head end knurled for providing torque resistance;

(0) a shank portion characterized by an uninterrupted series of radially projecting tiny annular barbs of constant root diameters and constant major diameters;

(d) the major diameters of said barbs being not more than .022 inch greater than the root diameters of the barbs;

(e) said barbs having an inclined approach surface, and a back surface approximately perpendicular to the longitudinal axis of the shank;

(f) said approach surface, in the direction toward the foot end of the insert, being inclined inwardly toward the center axis of the shank at an angle of between 15 and 25 relative to the shank axis.

2. A press insert of hardened metal for installation in a hole of softer metal, said insert comprising:

(a) a foot end having a pilot portion for insertion first into the hole;

(b) a head end knurled for providing torque resistance;

(0) a shank portion characterized by an uninterrupted series of radially projecting tiny annular barbs of constant root diameters and constant major diameters;

(d) said barbs having an inclined approach surface, and a back surface approximately perpendicular to the longitudinal axis of the shank;

(e) said approach surface, in the direction toward the foot end of the insert, being inclined inwardly toward the center axis of the shank at an angle of between 15 and 25 relative to the shank axis;

(f) said shank having a threaded bore for receiving a holding screw;

(g) the length of the insert, the total number of barbs, the diameter of the pilot portions, and the major diameter of the barbs being variable and related to the size of the holding screw in accordance with the following table:

Major Length diameter Installation of Pilot of hole Inter. thread insert, No. of diameter, barbs, diameter, size inch barbs inch inch inch References Cited UNITED STATES PATENTS 1,978,329 10/1934 Rosenberg -21 2,147,343 2/1939 Hokanson 85-21 2,304,036 12/1942 Tegarity 8521 2,967,448 10/ 1961 Hallock 1,5 l41.73 3,198,231 8/1965 Bisbing l5l-41.73 3,349,649 10/1967 Mele 15l-41.73

EDWARD C. ALLEN, Primary Examiner Powbo UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,498353 D d March 3, 1970 Invent0r(s) John K. Barry It is certified that error appears in the above-idemtifiedpatent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 20, "I have found by experiment that a good compromise" should read "I have found that the material of the insert should" SIGNED Mu EFTILEB momma.

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